The role of the baroreceptor reflex during hypertension is controversial. The proposed research will focus on three aspects of baroreflex function in hypertension: changes in receptor function in hypertension, baroreflex control in vasoactive hormone release in normo- and hypertension, and carotid sinus reflex control of vascular resistance and capacity functions in hypertension. The studies will be performed in hypertensive dogs with chronically isolated carotid sinus regions. Results from hypertensive dogs will be compared to normotensive controls. The possibility will be examined that a reduced compliance of the carotid sinus itself could contribute to a loss of functional baroreflex control in hypertension. The ability of the baroreflex to adapt to rapid changes in prevailing pressure, the rapid resetting process will be evaluated in hypertensive chronic dogs. The hypothesis will be tested that the changes in baroreflex function seen are occurring at the level of the receptor; measurement of afferent carotid sinus nerve activity will be used to assess changes in receptor-nerve characteristics. Inhibition of the carotid sinus by the action of the contralateral sinus will be assessed in hypertension. Carotid baroreflex release of vasoactive hormones will be studied in normo- and hypertensive dogs. The hypothesis to be tested is that increased release of these vasoactive hormones known to be released during physiological stresses contributes to the maintenance of elevated pressure levels in hypertension. Hormones to be studied include vasopressin, nor-epinephrine and B-endorphin. Baroreflex control of systemic vascular resistance will be examined. The entire systemic pressure-flow relationship will be measured and compared acutely in hypertensive dogs on cardiac bypass. In a further series of experiments, carotid baroreflex control of systemic vascular compliances will be measured. The hypothesis to be tested is that reduced vascular compliances and diminished reflex changes in compliance can account for the altered arterial pressure regulation in hypertension. In another series of proposed experiments, carotid sinus reflex control of systemic vascular capacity, and particularly unstressed vascular volume, will be assessed in hypertensive dogs on cardiac bypass. The final experiment will assess, both chronically and acutely, the differential changes in regional vascular resistance and capacity in hypertension. These experiments will provide new and important information regarding the mechanisms of afferent and efferent baroreflex control during hypertension.